Since fonts do not cover the entire Unicode space, you can provide a list of fallback fonts that will be tried for glyphs that are not available for a certain codepoint in the main selected font.

By default the system has a built-in cascade list that the system uses to satisfy the font. By setting this property is to set a list that is consulted before the system cascade list is looked up.

If you want to prevent the system cascade list from being consulted, you can use the special font name "LastResort", this is a special font that contains glyphs for every unicode code point. You can learn more about it at the Unicode web site: http://unicode.org/policies/lastresortfont_eula.html